The present invention is concerned with the manufacture of the read element in a magnetic disk system. This is a thin slice of material located between two magnetic shields. The principle governing operation of the read sensor is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance). In particular, most magnetic materials exhibit anisotropic behavior in that they have a preferred direction along which they are most easily magnetized (known as the easy axis). The magneto-resistance effect manifests itself as a decrease in resistivity when the material is magnetized in a direction perpendicular to the easy axis, said decrease being reduced to zero when magnetization is along the easy axis. Thus, any magnetic field that changes the direction of magnetization in a magneto-resistive material can be detected as a change in resistance.
Magneto-resistance can be significantly increased by means of a structure known as a spin valve (SV). The resulting increase (known as Giant magneto-resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of the solid as a whole.
The key elements of a spin valve structure are two magnetic layers separated by a non-magnetic layer. The thickness of the non-magnetic layer is chosen so that the magnetic layers are sufficiently far apart for exchange effects to be negligible (the layers do not influence each other's magnetic behavior at the atomic level) but are close enough to be within the mean free path of conduction electrons in the material. If the two magnetic layers are magnetized in opposite directions and a current is passed through them along the direction of magnetization, half the electrons in each layer will be subject to increased scattering while half will be unaffected (to a first approximation). Furthermore, only the unaffected electrons will have mean free paths long enough for them to have a high probability of crossing the non magnetic layer. Once these electrons have crossed the non-magnetic layer, they are immediately subject to increased scattering, thereby becoming unlikely to return to their original side, the overall result being a significant increase in the resistance of the entire structure.
In order to make use of the GMR effect, the direction of magnetization of one the layers must be permanently fixed, or pinned. Pinning is achieved by first magnetizing the layer (by depositing and/or annealing it in the presence of a magnetic field) and then permanently maintaining the magnetization by over coating with a layer of antiferromagnetic material. The other layer, by contrast, is a “free layer” whose direction of magnetization can be readily changed by an external field (such as that associated with a bit at the surface of a magnetic disk). Structures in which the pinned layer is at the top are referred to as top spin valves. Similarly, in a bottom spin valve structure the pinned layer is at the bottom.
Although not directly connected to the GMR effect, an important feature of spin valve structures is a pair of longitudinal bias stripes that are permanently magnetized in a direction parallel to the long dimension of the device. Their purpose is to prevent the formation of multiple magnetic domains in the free layer portion of the GMR sensor, particularly near its ends. Thus longitudinal bias is responsible for the stability of a spin-valve recording head. It is usually achieved by an abutted-type junction followed by hard bias and lead deposition.
Referring now to FIG. 1, we show there, in schematic cross-section, a top spin-valve device 11 which is resting on a substrate 14. Longitudinal bias leads 12 contact 11 along its sloping sidewalls 15 and are overlaid with conducting leads 13. To fabricate this device it is necessary to etch through the full thickness of device 11 during the formation of the abutted junction. This is to ensure that there will be enough hard bias material in contact with the free layer which, by definition, is near the bottom of the device.
In FIG. 2 we show a somewhat more detailed view of FIG. 1. Dielectric layer 23 (typically aluminum oxide) rests on magnetic shield layer 24. The spin valve (shown as 11 in FIG. 1) is made up of free layer 21, non-magnetic spacer layer 22, pinned layer 25, and antiferromagnetic (pinning) layer 26. As discussed above, it is necessary to expose as much as possible of free layer 21 if longitudinal bias leads 12 are to be their most effective. Unfortunately, this often results in the removal of a small amount 27 of dielectric layer 23, causing shorting to shield 24 (in areas such as 28) immediately or on life. As read heads grow ever smaller, it becomes necessary to reduce the thickness of dielectric layer 23 as much as possible so this shorting problem can only become worse.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,856,897, Mauri shows stabilization layers under the lead. In U.S. Pat. No. 6,208,491, Pinarbasi shows a SV with a capping structure. U.S. Pat. No. 6,185,078 (Lin et al.), U.S. Pat. No. 6,201,669 (Kakihara), and U.S. Pat. No. 6,208,492 (Pinarbasi) are all related patents.